1. Field of the Invention
The present invention relates to a heat-sensitive material using a diazonium salt and a coupling component as color developing components and, particularly, to a heat-sensitive recording material which is stable in light having longer wavelengths from about 350 nm of a light source typified by a fluorescent lamp, etc., and shows good storability before use and good density of color formation upon heating.
2. Description of the Related Art
A diazonium salt is a compound having a very high chemical activity, reacts with a compound called a "coupler", such as phenol derivatives, compounds containing an active methylene group, and the like, to easily form an azo dye. Also, such compounds possess photosensitivity and are decomposed by the irradiation of light, thereby losing their chemical activity. For these reasons, the diazonium salt has been utilized for a long time as an optical recording material typified by diazo copying (see Fundamentals of Photographic Engineering, Edition of Non-Silver Salt Photography, edited by Japan Photographic Association, published by Corona Co., Ltd., pp.89-117 and pp. 182-201(1982)).
Furthermore, the diazonium salt is recently applied to a recording material requiring image fixing by utilizing the property of loosing its activity by being decomposed by light. As a typical example, the so-called photo-fixing type heat-sensitive recording material wherein after forming images by heating a recording material provided with a recording layer containing a diazonium salt and a coupling component according to an image signal to cause a reaction of them, the images are fixed by the irradiation of light is proposed (see, Kohji Sato, et al., Journal of Image Electronic Society, Vol. 11, No. 4, pp.290-296(1982), etc.).
However, these recording materials using the diazonium salt as the color developing component have the drawback that the shelf-life as the recording material is short because the chemical activity of the diazonium salt is very high. This means that the diazonium salt is gradually decomposed even in the dark, thereby losing reactivity.
As the means for improving the instability of the diazonium salt described above, various methods are proposed and as one of the most effective means, there is a method of encapsulating the diazonium salt in microcapsules. With this method, the diazonium salt is isolated from materials which accelerate the decomposition thereof, such as water and bases. The decomposition of the diazonium salt is remarkably restrained and the shelf-life of the recording material using the microcapsules is greatly improved (see, Toshimasa Usami, et al., Journal of Electrophotographic Association, Vol. 26, No. 2, pp.115-125(1987)).
By microencapsulating the diazonium salt as described above, the stability as the heat-sensitive recording material can be greatly improved.
However, when the diazonium salt itself is chemically unstable, even when the diazonium salt is microencapsulated, there is a limit to the improvement in stability of the heat-sensitive recording material. To improve the stability of a heat-sensitive material, it is also important to improve the stability of the diazonium salt itself. In the conventional heat-sensitive recording material using a diazonium salt, after thermally printing, so-called fixing is carried out, that is, by irradiating the recording material thus printed with a light having the absorption wavelength of the diazonium salt, whereby the diazonium salt is photodecomposed to lose the reactivity with the coupling component. Accordingly, when the heat-sensitive recording material is allowed to stand for a long period of time in the light, the photodecomposition of the diazonium salt proceeds. This results in the problem that the density of color formation after storing is liable to fall and the like. In particular, in the case of preparing a multicolor heat-sensitive recording material by laminating plural heat-sensitive recording layers each containing a diazonium salt showing a different developed color hue from each other, there is a problem that when photo-fixing the diazonium salt in the upper layer, the uncolored diazonium salts existing in the lower layers are photodecomposed.